A variety of filling and repairing materials have been utilized to restore the function and configuration of a deficient part of a living body. Typical filling and repairing materials for living bodies include artificial bones and analogues such as artificial dental roots and crowns as well as artificial joints. They are generally known as living hard tissue replacements.
These living hard tissue replacements are required to be mechanically strong, tough, and stable in living bodies and should have high affinity to living bodies. Another important factor is ease of shaping because a living hard tissue replacement has to be a custom-made part conforming to an individual patient's deficient site.
The biological affinity used in this context means how a living hard tissue replacement adapts itself to and merges or assimilates with the surrounding living tissue where the replacement is embedded or implanted. Thus, a material having high biological affinity is scarcely recognized as xenobiotic by the surrounding tissue. Particularly when such material is used as an artificial bone, it can promote osteogenesis from the surrounding bone to eventually form a firm bond between itself and the bone tissue.
Among the currently available artificial bone materials, those featuring high mechanical strength and in vivo stability are metals such as titanium and zirconium, alloys containing such metals, and ceramics such as alumina, silicon nitride, and zirconia. The materials having high mechanical strength and in vivo stability, however, have low biological affinity, that is, are unlikely to assimilate with living tissue, resulting in an extended cure time and poor adherence to the living tissue. In addition, they have to be extracted and removed by surgical operations after they have performed their duties.
Typical of known materials having high biological affinity are bioglass, apatite (especially hydroxyapatite), tricalcium phosphate, and calcium phosphate crystallized glass. Apatite has the best biological affinity as understood from the fact that bone is essentially composed of apatite if organic components are excluded.
These materials having high biological affinity have low mechanical strength and toughness. Since normally a pressure of about 30 kg/cm.sup.2 and sometimes a maximum pressure of about 300 kg/cm.sup.2 is applied to the dental root during mastication, artificial dental roots of apatite seem unreliable in durability.
We discovered that among ceramic materials which contain CaO and SiO.sub.2 as essential components and optionally MgO, diopside, wollastonite and analogues, upon contact with body fluids, form a calcium phosphate base compound at the contact area so that these materials exhibit improved biological affinity, especially biological activity although they are non calcium phosphate ceramics. This is the subject matter of our preceding application, Japanese Patent Application No. 142058/1989 filed Jun. 6, 1989 (U.S. Ser. No. 07/533768 filed Jun. 6, 1990 or EPA 90110716.9 filed Jun. 6, 1990). These ceramic materials are suitable as living hard tissue replacements since they can compensate for the mechanical strength and toughness calcium phosphate base ceramic materials lack, without detracting from the biological affinity characteristic of calcium phosphate base ceramic materials.
In our preceding application, the ceramic materials are sintered at a temperature of 1,100.degree. to 1,550.degree. C. into a sintered body. In one preferred embodiment, the sintered body is pulverlized into a powder which is applied and joined to a substrate by spray coating, solvent welding, solution coating, or sputtering. To obtain a living hard tissue replacement having a complex profile, the sintered body has to be machined with difficulty. The preferred embodiment in which the ceramic material is bonded to a substrate has some likelihood that exfoliation can occur at the interface during repetitive use, indicating that the bonding force is less satisfactory. There are additional problems that the coating thickness is less uniform and shaping to a complex configuration is difficult.
It was believed in the prior art that ceramics can be shaped only by sintering. However, like metals, attempts have been made to subject ceramics to plastic forming, for example, by forging, extruding and rolling. For the plastic forming of ceramics, it is necessary to heat the material to a temperature higher than its melting point by about 60% or more, and the heating temperature can reach 2,000.degree. C. for a certain material. Nevertheless, those ceramic materials having superplasticity show extremely greater ductility, sometimes 10 times greater ductility, under low stresses at a temperature substantially lower, e.g., by 500.degree. C., than the sintering or forging temperature as described in the literature, for example, Journal of the JSTP, 29, 326 (March 1988); Ceramics, 24, 2 (1989); and Tetsu to Hagane (Iron and Steel), 75, 3 (1989). Typical prior art ceramic materials known to show superplastic nature are Y-TZP (yttria-stabilized tetragonal ZrO.sub.2 polycrystals) and ZrO.sub.2 -Al.sub.2 O.sub.3 systems. To take advantage of plastic deformation, extrusion molding and thin plate molding have been attempted on them. Attempts have also been made to diffusion bond two pieces of the same material by superplastic forming.
We have discovered that calcium phosphate base ceramic materials including apatite and tricalcium phosphate show superplastic nature. This is the subject matter of our application, Japanese Patent Application No. 155758/1989 filed Jun. 20, 1989 (U.S. Ser. No. 540926 filed Jun. 20, 1990.
However, no study has been made on the superplasticity of diopside. We have discovered that a superplastic phenomenon also develops with non-calcium phosphate base ceramic materials such as diopside.
We proposed in Japanese Patent Application No. 164959/1988 filed Jul. 4, 1988 (U.S. Ser. No. 374,989 filed Jul. 3, 1989 or EPA 89112220.2 filed Jul. 4, 1989), a sintered composite body having increased mechanical strength comprising a biologically active ceramic matrix such as apatite and inorganic whiskers dispersed therein. We have also discovered that this material can be subject to superplastic forming.